1. Field of the Invention
The invention relates to an electromagnetic contactor for an electric starter motor, said contactor comprising:
connection terminals intended for connection to the battery and to the electric motor,
a movable core,
a main stationary core,
an axial air gap provided between the movable core and the main stationary core,
a tubular coil to produce a magnetic current in the air gap provided between the movable core and a main stationary core during excitation,                a magnetic circuit provided with a case constructed with magnetic frame attached to the stationary core,        
an insulating cap enclosing the contacts of the electric power circuit and having connection terminals,                said case being composed of a metal bell-shaped housing, an internal ferrule made of magnetic material surrounding the coil, and a washer acting as an additional stationary core through which the movable core passes, and arranged opposite the main stationary core.        
2. Background Art
Starters generally comprise an electromagnetic contactor, the purpose of which is to make it possible for:                the pinion to engage in the drive ring gear at the moment of starting, as well as its disengaging after starting, and        the electric motor to be supplied with current.        
The contactor is generally composed of an electromagnet actuating a plunger core which, by moving, closes an electrical circuit for supplying power to the electric motor, and pulls an actuator lever that drives the pinion into the starter ring gear.
According to the document FR-A-2795884, a motor vehicle starter (see FIG. 5) has a rotary electric motor M and an output shaft equipped with a pinion 1 to drive a starter ring gear C, integral in rotation with the flywheel of the vehicle to start the combustion engine of the vehicle. The pinion 1 is slidably mounted, by means of complementary splines, on the output shaft between a rest position in which it is disengaged from the ring gear, and an active working position in which it engages with said ring gear.
The output shaft is driven in rotation by the electric motor when said motor is electrically powered. This shaft is different from the shaft of the motor M in FIG. 6 because speed-reduction gearing is located between the two shafts. As a variation, the output shaft is the shaft of the motor M.
The electric motor of the starter is also associated with an electromagnet power contactor 2 placed above the electric motor. This contactor 2 comprises a tubular coil 2a held by a support, the bottom of which constitutes a bearing 2c to guide a movable core 2b. 
This contactor 2 has the dual function of supplying the electric motor M with current, and of moving the movable pinion 1 between the two positions of rest and work. The excitation of the electromagnet is controlled, for example, by activating the contact key, which establishes the electrical circuit to the battery, after the closing of the contactor's main power circuit.
The movable core 2b of the contactor 2 is mechanically connected by a mechanical connection 4, comprising a control lever, to a starter drive assembly equipped with a freewheel transmission device. The pinion 1 pertains to the starter drive assembly.
The fork-shaped control lever is pivotably mounted on a spindle and the output shaft is mounted in rotation in a housing by means of bearings.
The housing is intended to be attached to a fixed part of the vehicle and is open for the passage of the ring gear C. This housing is thus used to attach the starter to the engine of the vehicle.
The main power circuit of the contactor is provided with a pair of fixed contacts, and a bridge-shaped movable contact 3 which is attached to a pushrod actuator intended to be moved in translation by the movable core during excitation of the coil.
More specifically, the pushrod is intended to be moved by the movable core 2b after the axial clearance is closed up, and a second return spring acts on the movable core to draw it back to the rest position.
A first return spring, called cutoff spring, pulls the movable contact and pushrod assembly to an open position in order to make an axial interval with the fixed contacts.
This rest position of the pushrod is determined by contact of the movable contact 3 with a stationary core having a central hole to guide the pushrod provided with a flange for mounting a spring, called contact pressure spring, acting between this flange and the movable contact. The stationary core is flanged and has a centering seat for centering the support of the coil 2a. 
Also provided is a spring 5, called gear engagement spring, housed inside the movable core 2b and engaged with a rod connected by a spindle to the upper end of the fork-shaped control lever to couple this lever to the movable core 2b. 
The contactor, generally cylindrical in shape, is situated near the electric motor while extending parallel thereto. It is attached to the above-mentioned housing that supports the output shaft and the pinion slidably mounted on said shaft. In a known way the housing also has the frame of the electric motor M closed by a rear bearing for the mounting in rotation of the shaft of the electric motor M. The housing has a front bearing for the mounting in rotation of the output shaft as an extension of the shaft of the electric motor M by means of a bearing between the ends of the said shafts.
In addition to the movable core, the contactor comprises a fixed part of magnetic material, and a cap of insulating material and having connection terminals connected to the fixed contacts. The fixed part of the contactor is composed of a dish-shaped frame designed to be mounted on the housing, and the stationary core separated from the movable core by an axial air gap. The tubular coil coaxially surrounds the movable core with a slight radial clearance, and is housed inside the case.
The movable core, under the action of the second return spring, is in a position separated from the stationary core when the coil is not excited.
When power is supplied to the coil, i.e., during excitation of the coil, the movable core moves by magnetic attraction toward the stationary core at first against the retraction force of the first return spring. After the closing up of the axial clearance between the movable core and the pushrod, the movable core then moves the pushrod against the force exerted by the second and first return springs. This first return spring is stiffer than the second return spring and is less stiff than the contact pressure spring.
This movement continues until the movable contact makes contact with the fixed contacts and power is supplied to the electric motor. The contact pressure spring is then compressed until the movable core comes into contact with the stationary core.
At the same time the starter drive assembly is moved, under action from the control lever, toward the ring gear C.
In the event the pinion 1 does not directly penetrate the ring gear C, the spring 5 is compressed to allow the fixed contacts to close and supply power to the electric motor, which then turns the pinion so that it can penetrate the ring gear C.
The structure of the magnetic circuit of such a contactor is well known, for example from the document DE 101 55 103 or by the above-mentioned document FR A 2 795 884.
In FIG. 1, the stationary core 10 of the contactor is generally immobilized in rotation with respect to the case 11 attached by one or more deformations of the case's side wall so as to form serrations 12 that are embedded in cavities 13 made on the periphery or behind the stationary core 10. When the case is produced by stacking several elements, careless handling of the case can result in a risk of the assembly coming apart.
In FIG. 2, the cap 14 must also be blocked from rotation with respect to the case 11 in order to withstand a certain tightening torque C on the connection terminals. The serrations 12a provided for that purpose are also made by deformation of the material of the end of the case 11, which are then inserted into the cavities 13a of the cap 14. When a certain tightening torque on the cap 14 is exceeded, said cap can undergo the beginning of rotation movement, with the risk of the serrations 12a escaping. This situation could occur in the event of insufficient mechanical rigidity of a case obtained by stamping a thin (0.5 to 1.5 mm) sheet of metal. The risk of ovalization of the case due to the action of a heavy rotation torque is then possible, and the function of immobilizing the case from rotation is no longer ensured.